The present invention relates to a porous honeycomb structure and a process for production thereof. More particularly, the present invention relates to a honeycomb structure balanced in purification ability, end face durability and canning property, and suitably used, for example, in a carrier for a catalyst for purification of automobile exhaust gas, as well as to a process for production thereof.
Porous honeycomb structures are in wide use in a catalyst carrier (e.g. a carrier for catalyst for purification of automobile exhaust gas, a filter (e.g. a filter for diesel engine exhaust gas), etc. The regulation for exhaust gas is becoming stricter year by year in consideration of environmental problems; in order to respond to such a trend, catalysts for exhaust gas purification, when used for purification of the exhaust gas emitted from an automobile engine or the like, are required to have an improved purification ability. Meanwhile, a lower fuel consumption and a higher output are aimed strongly from the standpoint of engine development; in order to respond to such a situation, catalysts for exhaust gas purification are also required to have a lower pressure loss. Hence, in order to satisfy these requirements, there is a strong movement that by allowing a honeycomb structure to have a very small wall thickness; the gas permeability is enhanced to reduce the pressure loss and also, weight of the catalyst for exhaust gas purification is reduced to reduce the heat capacity, to improve the purification ability during warm-up.
Meanwhile, various foreign matter may come into the exhaust gas emitted from an engine, whereby a honeycomb structure may be subjected to scraping-off (erosion); therefore, improvement in erosion resistance is also required.
The above improvement in purification ability (e.g. lower heat capacity and lower pressure loss) and the above improvement in erosion resistance are generally contradictory to each other. For example, when a honeycomb structure is allowed to have an increased porosity and a smaller wall thickness in order to achieve a lower heat capacity, the honeycomb structure comes to have a lower strength and a lower erosion resistance.
In such applications, the honeycomb structure is used by being held by a metal-made can or the like and, in such use, the honeycomb structure need be held at certain strength so that there occurs no slippage between the honeycomb structure and the can. An improvement in canning property of the honeycomb structure, such as strength capable of withstanding the above strength or an ability to prevent slippage even at a low holding strength, is required.
For improvement of erosion resistance, there are described, in, for example, JP-A-2000-51710, a honeycomb structure in which cell walls are made thick only in the vicinities of the honeycomb structure end face, and a honeycomb structure in which the end face is made dense by coating the end face of a base honeycomb structure with a vitreous component such as glaze, water glass or the like, followed by firing. In the literature, there is a description that the porosity of the end face is 5 to 35%; however, there is disclosed only a honeycomb structure in which the porosity of other portion is 36%. Such a honeycomb structure is unable to satisfy all of low heat capacity, erosion resistance and canning property. Further, the literature makes no mention of a relation between porosity, wall thickness and surface roughness.
For improvement of canning property, there is disclosed, in JP-A-2-207846, a honeycomb structure in which the surface of outer wall is allowed to have unevenness by a method of applying a vibration in extrusion of the honeycomb structure. However, this method has a problem in that the wall is cut off in extrusion when the wall is thin; therefore, the method is difficult to apply to a honeycomb structure having a thin wall. Also, in JP-A-2-86847 is proposed a honeycomb structure in which the surface of outer wall is allowed to have an uneven part by applying sandblasting or spraying a ceramic material to the surface and the uneven surface is made more rough than another surface; and a process for production of such a honeycomb structure. This process, however, needs an additional step and is not economical. In these literatures as well, no mention is made on a relation between porosity, wall thickness and surface roughness, and no investigation is made on erosion resistance or heat capacity reduction.
The present invention has been made in view of the above-mentioned problems and aims at providing a honeycomb structure which satisfies requirements of improved purification ability, erosion resistance and improved canning property at a good balance and which is suitably used particularly in a carrier for catalyst for purification of automobile exhaust gas, etc.
The present invention further aims at a process for producing such a honeycomb structure, particularly a honeycomb structure of low porosity and good canning property.
In order to achieve the above aims, the present inventors made a study including various tests. As a result, the present inventors found out that while, in general, a high porosity and a thin cell wall are required in order to achieve a low heat capacity and the opposite is required in order to obtain improved erosion resistance, the influence of porosity on erosion resistance is greater when the wall is thinner. The present inventors also found out that in achieving a certain heat capacity, good erosion resistance is obtained by making thinner the wall and lower the porosity. The present inventors also found out that when the porosity is controlled at less than 30%, the canning property is reduced but this is caused by the smoothening of the surface of outer wall. The present inventors also found out that the canning property can be improved by allowing the average surface roughness (Ras) to fall in 0.5 xcexcmxe2x89xa6Rasxe2x89xa610 xcexcm and/or allowing the average height (RzDINs) on the surface of outer wall from a local maximum point to a next local minimum point to fall in 5 xcexcmxe2x89xa6RzDINsxe2x89xa650 xcexcm, and further that the above Ras and RzDINs can be achieved by allowing the pores having pore diameters of 1 xcexcm or more to be 90% by volume or more of the total pore volume. The first aspect of the present invention is based on the above findings.
The first aspect of the present invention provides a honeycomb structure constituted by
porous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, and
a honeycomb outer wall that surrounds and holds the outermost peripheral cells located at a circumference of the cell group,
characterized in that a porosity is 5% to less than 30%, a basic wall thickness (Tc) of the cell walls is 0.030 mmxe2x89xa6Tc less than 0.076 mm, an average surface roughness (Ras) of the outer wall is 0.5 xcexcmxe2x89xa6Rasxe2x89xa610 xcexcm, and/or an average height (RzDINs) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 xcexcmxe2x89xa6RzDINsxe2x89xa650 xcexcm.
In the first aspect of the present invention, a preferred example is a honeycomb structure having the following characteristics. That is, in the first aspect of the present invention, it is preferred for improved canning property that Ras is 0.5 xcexcmxe2x89xa6Rasxe2x89xa610 xcexcm and RzDINs is 5 xcexcmxe2x89xa6RzDINsxe2x89xa650 xcexcm and, preferably, Ras is 1.0 xcexcmxe2x89xa6Rasxe2x89xa610 xcexcm and RzDINs is 10 xcexcmxe2x89xa6RzDINsxe2x89xa650 xcexcm, and an open frontal area (P) of honeycomb structure, of 83% or more is preferred for lower heat capacity, lower pressure loss and improved purification ability. Further, an outer wall thickness of 0.076 mm or more is preferred for improved canning property. Also, it is preferred that the average surface pore diameter of outer wall is larger than an average inside pore diameter of outer wall, in order to make Ras and RzDINs large. It is also preferred for higher loading power for catalyst that the average surface roughness (Rac) of the cell wall is 0.5 xcexcm or more and/or the average height (RzDINc) on the surface of the cell wall from a local maximum point to a next local minimum point is 5 xcexcm or more. It is further preferred that in the honeycomb structure, the pores having pore diameters of 1 xcexcm or more is 90% or more of the total pore volume, in order to make large Ra and RzDIN.
In order to obtain improved erosion resistance, improved isostatic strength and improved canning property, it is preferred that there is a relation of 1.10xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa63.00, preferably, 1.10xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa62.50, more preferably 1.20xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa61.60 between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr5-20) of cells existing between the outermost peripheral cell taken as a first cell and also as a first starting cell and a first end cell which is any cell of a fifth cell to a twentieth cell, preferably a fifth cell to a fifteenth cell, all extending inwardly from the first starting cell. In that case, it is preferred that, taking a first cell inward from and adjacent to the first end cell as a second starting cell, cells existing between the second starting cell and a second end cell, which is any cell of a third cell to a fifth cell all extending inwardly from the second starting cell, have such a cell wall thickness that is substantially uniform, inverse trapezoid or spool-shaped in each cell and gets thinner one by one inwardly and that the thinnest cell wall thickness is identical to the basic cell wall thickness (Tc).
It is also preferred that there is a relation of 1.10xe2x89xa6Tr1/Tcxe2x89xa63.00 between the cell wall thickness (Tr1) of outermost peripheral cell and the basic cell wall thickness (Tc), taking outermost peripheral cell as a third starting cell, there is a relation of 1.10xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa63.00, preferably 1.10xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa62.50, more preferably 1.20xe2x89xa6(Tr1xcx9cTr5-20)/Tcxe2x89xa61.60 between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr5-20) of cells existing between the third starting cell and a third end cell which is any cell of a fifth cell to a twentieth cell all extending inwardly from the third starting cell, the cells have a cell wall sectional shape of inverse trapezoid or spool or have a uniform cell wall thickness and their cell wall thicknesses get smaller as the cell is more inward, and the smallest cell wall thickness is identical to the basic cell wall thickness (Tc).
When the honeycomb structure has a sectional area of 160 cm2 or more, it is preferred that there is a relation of 1.10xe2x89xa6(Tr1xcx9cTr10-40)/Tcxe2x89xa63.00, preferably 1.10xe2x89xa6(Tr1xcx9cTr10-40)/Tcxe2x89xa62.50, more preferably 1.20xe2x89xa6(Tr1xcx9cTr10-40)/Tcxe2x89xa61.60 between the basic cell wall thickness (Tc) and each cell wall thickness (Tr1xcx9cTr10-40) of cells existing between the outermost peripheral cell taken as a first cell and also as a first starting cell and a first end cell which is any cell of a tenth cell to a fortieth cell all extending inwardly from the first starting cell.
It is further preferred for improvement of erosion resistance of honeycomb structure end face that the value of porosity (%) of part or all of the cell wall portions within 30 mm from one or both end faces of the honeycomb structure is lower by at least 5 than the value of porosity (%) of other cell wall portion. It is also preferred for balancing of erosion resistance and low heat capacity that the length of the cell wall portions having a lower porosity, from the honeycomb structure end face is not uniform. It is also preferred for improved erosion resistance that the thickness of the cell walls at one or both end faces of the honeycomb structure is larger than the thickness of cell walls at other portion.
Thus, by making large the thickness of the cell wall near the circumference of the honeycomb structure and/or making strong the end face, the basic wall thickness (Tc) can be made even smaller. Tcxe2x89xa60.056 mm is preferred for lower heat capacity.
Further, the honeycomb structure of the present invention preferably has a B axis flexural strength of 0.8 MPa or more and is preferably made of at least one kind of material selected from the group consisting of cordierite, alumina, mullite, silicon nitride, aluminum titanate, zirconia and silicon carbide. The honeycomb structure of the present invention preferably has a sectional shape of circle, elliptic, oval, trapezoid, triangular, tetragonal, hexagonal or asymmetry between the right and the left, and its cell sectional shape is preferably triangular or hexagonal. It is also preferred that the present honeycomb structure is formed so that an intersection between cell walls has a radius of curvature of 1.2 mm or less. It is also preferred that an intersection between cell wall and a honeycomb outer wall has a radius of curvature of 1.2 mm or less.
The honeycomb structure of the present invention is preferably used as a carrier for catalyst for purification of automobile exhaust gas. It is also preferred that a catalyst component is loaded on the cell walls of the honeycomb structure, which is assembled into a catalytic converter, and held the outer wall at the outer surface.
The present inventors investigated on the process for producing a honeycomb structure having the above-mentioned properties. As a result, the present inventors found out that by using, as a raw material, a combination of talc and kaolin both having particular particle diameters, a honeycomb structure having a rough surface and a low porosity can be produced economically.
The second aspect of the present invention provides a process for producing a honeycomb structure constituted by porous cell walls which form a cell group consisting of a plurality of cells adjacent to each other, and a honeycomb outer wall which surrounds and holds the outermost peripheral cells located at a circumference of the cell group, in which honeycomb structure a porosity is 5% to less than 30%, a basic wall thickness (Tc) of the cell walls is 0.030 mmxe2x89xa6Tc less than 0.076 mm, an average surface roughness (Ras) of the outer wall is 0.5 xcexcmxe2x89xa6Rasxe2x89xa610 xcexcm, and/or an average height (RzDINs) on the surface of the outer wall from a local maximum point to a next local minimum point is 5 xcexcmxe2x89xa6RzDINsxe2x89xa650 xcexcm, the process being characterized in that production is conducted using a raw material containing at least a talc component and a kaolin component, the talc component containing coarse particle talc having an average particle diameter of 7 xcexcm or more and fine particle talc having an average particle diameter of ⅔ or less of that of the coarse particle talc, the kaolin component containing coarse particle kaolin having an average particle diameter of 7 xcexcm or more and fine particle kaolin having an average particle diameter of ⅔ or less of that of the coarse particle kaolin.
In the second aspect as well, a preferred honeycomb structure is a honeycomb structure having preferable characteristics mentioned in the first aspect.